The disclosures of these co-pending applications are incorporated herein by cross-reference.
The present invention relates to materials potentially suitable for use as the expansive element in thermoelastic design and to methods for ranking the potential relative suitabilities of those materials.
The invention as developed originally as a means of identifying and ranking a range of materials that potentially may exhibit superior properties for use in the manufacture of microscopic thermal bend actuators for use in micro-electro mechanical systems (MEMS), and will be described hereinafter with reference to this field. However, it will be appreciated that the invention is not limited to this particular use and is equally applicable to macroscopic design even though the overall design considerations are vastly different and certainly less complex.
It is important to clarify that thermoelastic actuation is characterized using force, deflection and temperature as opposed to switching, which is characterized using deflection and temperature rise alone. Macroscopic thermoelastic actuators are typically used as switches that activate other more energy efficient actuation systems, however, microscopic thermoelastic actuators are an attractive actuation mechanism for a number of reasons. This includes the down scaling of certain physical phenomena. For example, it is possible to fabricate very thin films that decrease the thermal mass and minimize efficiency losses. Opposing gravitational and inertial forces become negligible on the microscopic scale. Other advantages include ease of fabrication (although more complex than simple electrostatic actuators) and the possibility of low voltage operation. Disadvantages include a low operational bandwidth determined by the thermal conductivities of substrate materialsxe2x80x94this is more of an advantage for the current application allowing for rapid firing.
A relatively diverse range of output force and deflection values can be obtained by altering actuator geometry. However, the fundamental operation of actuation is directly related to the mechanical and thermal properties of the component materials. Correct material selection in association with effective design can result in either a smaller or a more efficient actuator. Such an actuator increases wafer yield and is thus more commercially viable. A more efficient actuator may be battery powered increasing operation simplicity and negating the requirement for expensive voltage transformers. An increase in thermal efficiency improves the operational firing frequency, and decreases the possibility of thermal crosstalk. This is especially relevant for arrays of thermal actuators in a micro-cilia device.
However, material selection for MEMS application is not straightforward. Firstly, published thin film properties can vary greatly due to different fabrication methods and difficulties associated with experimentally quantifying material properties on the microscopic scale. Secondly, certain thin films can only be fabricated with certain layer thicknesses because inherent stress can shatter or curl the substrate wafer. Thirdly, only certain materials can be used in the fabrication process at most fabs as the introduction of a new material can contaminate machinery.
Progress to Date
Until recently, the only materials regularly used or considered for use in such applications were polysilicon, single crystal silicon. However, the applicant just previously made the surprising discovery that titanium nitride and titanium boride/diboride exhibited excellent properties relevant to this application.
Realizing the breakthrough this surprising discovery signified, the applicant sought to try and identify possible alternatives in order to provide designers of thermoelastic systems with more choice and flexibility. However, given the lack of available data on their film properties for various materials and the fact that empirical testing with MEMS would be prohibitively expensive, there was clearly a need, or it was at least highly desirable to be able to determine a method of evaluating materials for this use based solely on the commonly available macro material properties.
It is therefore an ultimate object of one aspect of this invention to identify a range of alternative materials that will potentially exhibit superior properties for use in thermoelastic design and of another aspect to provide a means of ranking the potential suitability of a given range of materials for this same use.
According to a first aspect of the invention there is provided a method of selecting a material for use as the expansive element in a thermoelastic design by deriving an indicator of the material""s potential effectiveness for that use, said method including the step of calculating a dimensionless constant xcex5xcex3 for that material in accordance with the formula:       ϵ    ⁢          xe2x80x83        ⁢    γ    =            E      ⁢              xe2x80x83            ⁢              γ        2            ⁢      T              ρ      ⁢              xe2x80x83            ⁢      C      
wherein E is the Young""s modulus of the material; xcex3 is the coefficient of thermal expansion; T is the maximum operating temperature, xcfx81 is the density and C is the specific heat capacity.
In accordance with a second aspect the invention, in another broad form, also provides a method of manufacturing a thermoelastic element that includes at least one expansive element, the method including:
selecting a material for use as the expansive element in the thermoelastic design by deriving an indicator of the material""s potential effectiveness for that use, said method including the step of calculating a dimensionless constant xcex5xcex3 for that material in accordance with the formula:       ϵ    ⁢          xe2x80x83        ⁢    γ    =            E      ⁢              xe2x80x83            ⁢              γ        2            ⁢      T              ρ      ⁢              xe2x80x83            ⁢      C      
wherein E is the Young""s modulus of the material; xcex3 is the coefficient of thermal expansion; T is the maximum operating temperature, xcfx81 is the density and C is the specific heat capacity and selecting the material on the basis of xcex5, and
manufacturing the thermoelastic element with the at least one expansive element formed of the selected material.
Preferably, the method of selection includes the step of normalizing the dimensionless constant relative to that of silicon to a value xcex5 which is achieved by deriving the value xcex5xcex3 for the material of interest at the relevant temperature value and dividing this by the value of xcex5 obtained for silicon at that same temperature.
The relevant maximum operating temperature will depend upon the surrounding materials and their function but is most commonly the oxidizing temperature or the melting point temperature.
Desirably, the selection method includes the step of eliminating certain materials by requiring a pre-determined resistivity range. In one preferred form this resistivity range is between 0.1 xcexcxcexa9m and 10.0 xcexcxcexa9m.
In accordance with a third aspect of the invention there is provided an expansive element in a thermoelastic design that is made from any functionally suitable material or combinations of materials selected from a group including:
silicides and carbides of titanium.
In accordance with a fourth aspect of the invention there is provided an expansive element in a thermoelastic design that is made from any functionally suitable material or combinations of materials selected from a group including:
borides, silicides, carbides and nitrides of tantalum, molybdenum, niobium, chromium, tungsten, vanadium, and zirconium.
In accordance with a fifth aspect of the invention there is provided an expansive element in a thermoelastic design that is made from any functionally suitable alloy material or combinations of alloy materials selected from the group including:
borides, silicides, carbides and nitrides of titanium, tantalum, molybdenum, niobium, chromium, tungsten, vanadium, and zirconium.
Preferably the expansive element in a thermoelastic design in accordance with the third, fourth or fifth aspect of the invention also includes one or more of the following properties:
(a) a resistivity between 0.1 xcexcxcexa9m and 10.0 xcexcxcexa9m;
(b) chemically inert in air;
(c) chemically inert in the chosen ink; and
(d) depositable by CVD, sputtering or other thin film deposition technique.